1. Field of the Invention
The present invention relates to a transfer fixing device for transferring and fixing an unfixed image (toner image etc.) formed on an image bearing and conveying body such as an intermediate transfer belt onto a recording medium, the image bearing and conveying body, and an image forming apparatus, and particularly to an improvement of a transfer fixing device, an image bearing and conveying body, and an image forming apparatus, which are effective when a recording medium provided with a thermoplastic resin layer on its surface is used.
2. Description of the Related Art
In recent years, with the development of a computer and the improvement of a network or the like, and the popularization of a scanner, digital camera, or the like in addition to the appearance of a mass storage medium, photo-like image data have been rapidly spread and a demand to print out these image data with high picture quality has been increased. Particularly, a demand to improve the picture quality by putting a gloss on a full color image has been increased.
On the other hand, in an image forming apparatus such as a copying machine of an electrophotographic system or the like, a printer, or a facsimile, as a transferring and simultaneous fixing system in which transferring and fixing are simultaneously carried out by instananeously heating and pressing a toner image has been technically investigated, it has been attempted to increase the speed of image formation.
Here, as this kind of transfer fixing device, for example, there is widely known a device in which a toner image is formed on an image bearing formation body such as a photoreceptor drum, the toner image is transferred onto an image bearing and conveying body such as an intermediate transfer belt, and then, instantaneous heat and pressure is imparted to the toner image on the intermediate transfer belt by a heating roll and a pair of pressure rolls, so that the toner image is transferred and simultaneously fixed onto a recording medium.
However, the toner image does not entirely permeate a paper sheet as a general recording medium, but is formed to be swelled on the recording medium. Thus, in a halftone region or highlight region, the structures of line screen or dot screen are swelled and are unevenly swelled from the recording medium, so that scattering of incident light becomes high. In the case where a picture image having relatively high concentration gradation, such as a figure image, is formed, the picture image in which a high glossy region and a low glossy region are mixed, is obtained and gives a feeling of wrongness. Further, in such a rough picture image, color reproducibility is lowered by the influence of diffused reflection on the surface of the picture image, and it becomes a picture image with low vividness.
As a method for solving such a technical problem, there is already disclosed a method in which a transparent resin layer is provided on a recording medium, a toner is transferred onto the recording medium, and then, the toner is embedded into the transparent resin layer by a roll heat fixing machine (for example, Japanese Patent Unexamined Publication No. Sho. 63-92965), or a method in which a thermoplastic transparent resin layer is provided on a recording medium, and the transparent resin having the softening point (Tmp) within the range from +10xc2x0 C. to xe2x88x9230xc2x0 C. with respect to the softening point (Tmt) of the toner is used, so that a picture image superior in image gloss and color reproducibility, without formation of irregular color, superior in graininess, and superior in uniformity of image gloss is obtained (for example, Japanese Patent Unexamined Publication No. Hei. 10-63028).
However, even if this kind of recording medium, that is, the recording medium having the uppermost layer covered with the resin layer in which toner is embedded is used, and the toner image is transferred and fixed by the foregoing transfer fixing device, when the transferring/fixing speed, that is, the moving speed of the intermediate transfer belt is increased, heat supply to the toner image and the thermoplastic resin layer is not sufficiently performed and becomes insufficient. As a result, the toner image comes not to sink into the thermoplastic resin layer, so that the surface smoothness of the picture image formed on the surface of the recording medium is lowered, and image irregularity or minute gloss irregularity becomes liable to occur. Particularly, in the halftone region, since heat supply to the thermoplastic resin layer through the toner becomes insufficient, there is seen a phenomenon in which the influence remarkably appears.
In order to avoid such a state, in the foregoing transfer fixing device, even if a method of raising the temperature of the heating roll for supplying heat to the toner image and the thermoplastic resin layer is used, in the case where fixing is made at high speed, since a time when the thermoplastic resin layer of the recording medium is in contact with the heating roll is short, a rise in heat supply quantity becomes slight, the thermoplastic resin layer is not sufficiently melted, and sinking of the toner image is not improved. If the temperature of the heating roll is excessively raised, there is also a fear that toner is excessively melted and a hot offset occurs.
Then, there is proposed a technique in which a recording medium is preheated before transferring and fixing, and a thermoplastic resin layer of the recording medium is made sufficiently melted at the time of the transferring and fixing, so that the degree of sinking of a toner image is improved even at high speed.
Conventionally, as this kind of preheating system, there is known an image forming apparatus in which a toner image on an image bearing formation body is once pressed and transferred onto an intermediate transfer belt, and this is transferred onto a recording medium at a transfer fixing portion formed of a heating roll and a pressure roll and is simultaneously heated and fixed, and in which a heating body such as a plate-like heating member is disposed to be close to or in contact with the pressure roll, and the recording medium is made to go into the transfer fixing portion while being heated by the hating body such as the plate-like heating member (for example, see Japanese Patent Publication No. Sho. 64-698, No. Hei. 2-61749).
However, in the prior art of this kind, since the heating body such the plate-like heating member is provided in a recording medium conveying passage and the recording medium is preheated by this heating body, there is a fear that in the resin coated recording medium, the surface resin layer adheres to a guide shoot of the recording medium conveying passage or the like to increase conveying resistance, a jam or the like occurs, and it becomes impossible to convey the recording medium.
There is also proposed a method in which the foregoing heating body is not used, but a recording medium is guided to a press contact position along a peripheral surface of a heating roll by a guide member (see Japanese Patent Publication No. Sho. 63-50711). However, this method has a similar technical problem in that the preheated and melted surface resin layer is liable to adhere to the guide member.
As a non-contact type preheating system, it is conceivable to heat a recording medium by light of a halogen lamp or the like (for example, Japanese Patent Unexamined Publication No. Hei. 5-173448). However, since the recording medium is white or transparent, the light absorption efficiency is very poor in a normal system, and it is not practical.
If an industrial hot air heater used for film weld or the like is employed as it is, there occurs a technical problem of poor running of a recording medium due to curl of the recording medium, partial adhesion of the surface resin layer, or the like.
On the other hand, there is disclosed a method in which in order to secure a sufficient transferring/fixing time at high speed transferring/fixing, a heating endless belt is used instead of the heating roll (for example, Japanese Patent Unexamined Publication No. Hei. 7-49622).
In this method, the heating endless belt is disposed to come in contact with an intermediate transfer belt in a wide range, a recording medium is made to pass between the intermediate transfer belt and the heating endless belt, and a toner image on the intermediate transfer belt is transferred and fixed onto the recording medium by using the heating endless belt, so that a sufficient transferring/fixing time is secured even at high speed.
However, in this type of transfer fixing device, since heating can be made only from the reverse side (non-image bearing surface side) of the recording medium, in order to supply heat sufficient to melt the toner image and the thermoplastic resin layer, it is necessary to heat the heating endless belt to a high temperature.
In this case, since the temperature of the thermoplastic resin layer becomes higher than, the toner temperature, in the case where the set temperature of the heating endless belt is high, the thermoplastic resin layer is excessively melted, and the toner is diffused, so that the sharpness of a picture image is deteriorated, or the melting viscosity of the surface resin layer of the recording medium is excessively lowered, so that poor peeling becomes liable to occur. If the set temperature of the heating endless belt is made low, the toner comes not to be melted sufficiently, and there occurs a technical problem that a coloring property becomes poor.
As a method of solving such technical problems, there is proposed a technique in which an image forming apparatus is provided with such a transfer fixing device that an image bearing and conveying body (for example, an intermediate transfer belt) for bearing and conveying an unfixed toner image is disposed to come in contact with a recording medium provided with a thermoplastic resin layer on its surface, and the unfixed toner image on the image bearing and conveying body is made to be transferred and fixed onto the thermoplastic resin layer of the recording medium, and in which the transfer fixing device includes at least three rolls for heating and pressing the toner image nipped between the image bearing and conveying body and the recording medium, and at least two rolls of these rolls are disposed at positions where they nip the image bearing and conveying body, the toner image, and the recording medium therebetween, and are in contact with the remaining one roll (for example, see Japanese Patent Unexamined Publication No. Hei. 11-52760).
By this, even if transferring and fixing is performed at high speed, the gloss of the surface of a picture image is high, and the occurrence of minute gloss irregularity is prevented.
However, in the transfer fixing device of the prior art, there has been found such a technical problem that since the reverse side of the recording medium is also heated to the same degreed as the obverse side, when the recording medium is continuously made to pass in a continuous print mode or the like, the temperature of the reverse surface of the recording medium is raised, and a so-called blister phenomenon in which moisture in the inside of the recording medium is transformed into bubbles occurs, which causes a picture quality defect.
Besides, in the conventional image forming apparatus of the transferring and simultaneous fixing system, there is also such a technical problem that since the surface state of the image bearing and conveying body such as the intermediate transfer belt is normally formed to be uniform, a formed image is inevitably limited to one gloss, and it is difficult to obtain a gloss desired by the user.
There is an example in which in order to improve the quality of a projection image for an OHP sheet, the surface of an image bearing and conveying body is divided into one for a plain paper and one for the OHP sheet (for example, see Japanese Patent Unexamined Publication No. Hei. 11-24430). In this example, transmission linearity of light to the OHP sheet is considered, and particularly, there is a factor of total reflection which the user does not desire relatively, so that it is unsatisfactory in the meaning of selecting the gloss desired by the user.
The present invention has been made in view of the above circumstances and provides a transfer fixing device, an image bearing and conveying body, and an image forming apparatus, in which a picture image with an excellent gloss can be obtained even if a thick resin coated recording medium with high heat capacity is used.
More specifically, a first aspect of the present invention provides a transfer fixing device and an image forming apparatus in which even if a thick resin coated recording medium with high heat capacity is used and transferring/fixing is performed at high speed, a fixed image sinks sufficiently into a thermoplastic resin layer of the recording medium and plane smoothness of the fixed image is secured, and high picture quality without defects in picture quality, such as a blister phenomenon, can be maintained.
A second aspect of the present invention provides an image bearing and conveying body and an image forming apparatus in which even if a thick resin coated recording medium with high heat capacity is used, a gloss desired by a user can be easily obtained.